Field of the Invention
This invention relates generally to High-Rate Overcharge-Protection Separators for Rechargeable Lithium-Ion Batteries.
Brief Description of the Related Art
With the growing demand for hybrid and electric vehicles, the estimated market for high-performance lithium-ion batteries will total more than $7 billion annually by 2015. Typical hybrid vehicles contain between 50 and 70 battery cells; plug-in electric vehicles with range-extending motors have 80 to more than 200 cells; and fully electric vehicles can carry thousands of cells. A typical lithium-ion battery cell has a transition metal oxide positive electrode and a carbon based negative electrode. An electronically insulating yet ionically conducting separator membrane is placed between the two electrodes to prevent them from touching and shorting while allowing lithium ions to pass back and forth during the charge and discharge of the battery.
Non-aqueous electrochemical cells in such arrangements suffer from safety and lifetime issues upon overcharge and/or overdischarge. The term “overcharge” is used to describe a variety of conditions, including simple charging at normal rates beyond rated capacity, overvoltage excursions for short or long periods, charging at a rate too high for one electrode (commonly the anode) without exceeding the maximum voltage, and other more complex scenarios. The damage to the overcharged cells may include electrode degradation, electrolyte breakdown, current collector corrosion, and gas evolution. Li and dissolved transition metals may plate on the anode, which can lead to the formation of internal shorts and consequently hazardous conditions. Moreover, even slight overcharging reduces the discharge capacity of a cell, which can result in over discharging, increased impedance, local heating, etc., and jeopardize the lifetime of the battery cells. As the battery industry moves towards lithium cells with higher energy density and larger packs for vehicular applications, there is paramount importance of preventing catastrophic failures and safety issues due to cell overcharge/overdischarge.
Battery packs for consumer electronics are typically overcharge-protected by external electronic controls, shutdown components, or redox shuttles. The electronics can monitor each cell and disconnect it acting in response to voltage, temperature or pressure, but they add a substantial amount of weight, cost and complexity to the battery pack. The shutdown mechanism operates by dramatically increasing the internal resistance upon the excursions of voltage, temperature or pressure, and permanently deactivates the overcharged cell. The commonly used shutdown methods involve the use of a battery separator having a central component that melts at ˜160° C., an additive that produces gases, or an additive that polymerizes to form insulating compounds. In a multi-cell stack capable of delivering several hundred volts, permanently shutting down a cell reduces the usable capacity of the stack and puts added strain on the remaining cells in parallel with it. Other approaches, like system over sizing or complex re-routing of current around overcharged cells, are also impractical in these stacks due to cost and weight issues.
Reversible internal protection mechanisms that maintain a cell's potential and discharge capacity can provide protection without the above-mentioned disadvantages. Redox shuttles are able to balance the cells in the string and allow for continuous operation in the event of overcharge, but their reliance on molecular and cationic diffusion in the electrolyte limits the sustainable current density and cell charging rate. This approach also fails to work at low temperatures that are commonly experienced by vehicles. The most promising method is to use a self-actuated component that acts as a reversible electrochemical switch regulated by cell voltage. In the event of overcharge and/or overdischarge, the component creates a current bypath to protect the cell against catastrophic failure. Unlike the redox shuttle method, it conducts overcharge current through an electronic rather than a diffusion path, and therefore is capable of high-rate and low-temperature protections. The concept was first disclosed in U.S. Pat. No. 6,228,516 (May 2001), and later experimentally demonstrated in our work in Chen et. al., Electrochemical and Solid State Letters, 2004, 7(2), A23-26, the contents of which are herein incorporated by reference in its entirety. In our initial studies, a commercial micro porous separator was impregnated with poly (3-butylthiophene) also known as P3BT, an electrochemically active polymer that becomes electronically conducting when oxidized at 3.2 V. The composite membrane was introduced into a TiS2-Li cell in place of the regular micro porous separator, which transformed the battery into a resistor at the triggering voltage and limited the charging potential at 3.2 V to prevent the cell from overcharge damage.
In further detail, a self-switching bypass structure for a Li-ion cell was made by coating a voltage activated conductive polymer (VACP), poly(3-butylthiophene) (P3BT), onto a conventional micro-porous polyethylene separator. By this method, the VACP is dissolved in a solvent such as chloroform to form a low viscosity solution. The solution is coated on both sides of a commercial polyethylene (PE) or polypropylene (PP) micro-porous separator (Celgard 2500). The solution flows into and through the preexisting pores of the polyolefin separator. When the chloroform evaporates it leaves behind a film of VACP on the surface of the separator and a network of solid VACP that has penetrated the existing pores of the separator to connect the two, coated faces of the separator. The use and effect of the VACP coated separator is similar to a standard external electronic bypass circuit though potentially less expensive and more responsive to overcharge conditions. A Li-ion cell was made using a standard LiMn2O4 cathode and carbon anode laminates with the VACP coated separator sandwiched in between. The VACP coated separator became electrically conductive to generate a short between the anode and cathode electrodes when the cell voltage exceeded the conductive onset voltage of the VACP material, in this case approximately 3.4 V. Thus the cell could not be charged beyond this point, preventing cell overcharge or potentially allowing for cell balancing in strings of cells. In this initial work, the maximum bypass current achieved was approximately 0.2 mA/cm2, above which the cell voltage would continue to rise. On issue related to this particular method is that the coating process results in the bulk of the VACP being present on the surface of the two faces of the separator film where it contributes very little to the current bypass capability of the separator. Because VACP materials are typically more expensive than the materials used to make the separator, it is preferred that the amount of the VACP material be minimized.
Substantial DOE financial support was obtained in further developing this technology, including an SBIR collaboration with Farasis Energy, Inc. In August 2011, Farasis Energy Inc. filed a patent (U.S. Pat. No. 7,989,103) disclosing methods of making micro porous separators incorporating a voltage activated conductive polymer.
A number of issues remain in the current state-of-the-art overcharge-protection separators. The micro porous composite separators typically have lower porosity compared to the conventional ones, which leads to higher cell impedance and lower power density. Non-uniform distribution of the electroactive polymer in the composite membrane can also lead to low utilization, high internal resistance, localized heat generation and instability. Moreover, high loading of the relatively expensive electroactive polymer and processing complexity largely limit the scale-up options and consequently industrial adaptation of the technology.
An alternative to the state-of-the-art commercial micro porous separators are porous non-conducting fine-fiber separators as disclosed by DuPont in U.S. Pat. No. 7,112,389 (September 2006) the contents of which are herein incorporated by reference in its entirety. Owning to its unique pore structure and large porosity, the membranes can provide electronic and dendritic barriers in secondary lithium-ion batteries with reduced thickness and lower ionic resistance, which enables faster charge and discharge for increased power density. Low cost and scalable fiber spinning methods are available to assemble fibrous polymer mats composed of fiber diameters ranging from several microns down to lower than 50 nm. This type of non-conducting fiber membrane is promising as the next generation separators for rechargeable batteries.
Notwithstanding these results, there still remains a need for electroactive conducting fiber membranes for use as high-rate overcharge-protection separators for rechargeable lithium-ion batteries.